Various electronic packaging methods, implemented to provide high I/O as well as increased packing density, have imposed new requirements on the performance characteristics of inspection systems. These methods include the controlled collapse connections of Flip Chip Attach as well as Ball Grid Array (BGA) and Solder Column Connect technologies, the former employing high density solder ball connections and the latter employing a similar array but of columnar leads. "BGA", as used in this application, refers to a genus of packaging strategies employing varying sizes and shapes of leads which need not be restricted to actual spheres of solder but may include columns or mounds as well. Characteristic of the various technologies is the significance, to acceptability for assembly, of the three-dimensional shape of the lead structure.
For reliability of the BGA solder connections, is important that there be neither too much nor inadequate solder, and that the balls be present, unshorted, and properly registered, both laterally with respect to the pad array structure and vertically so as to ensure coplanarity of the points of tangency to the entire array of balls. Furthermore, it is important that inspection of the bonding structures be performed rapidly and accurately.
Methods of lead inspection which are known in the art include x-ray inspection, triangulation to determine the point of optical reflection of a point source of light synchronously scanned across the array, and focussing, by means of multiple exposures, of an array camera on the BGA to determine a focal position for each ball. Existing techniques requiring multiple image acquisitions lack the efficiency of a method whereby three dimensional information regarding the entire array is determined in a single exposure.
Other applications, such as those in which parts employing small ball bearings are inspected, for example, similarly require accurate determination of the three-dimensional structure of a plurality of small components within a single "view" of the part.